1. Field of the Invention
The present invention relates to an antenna unit and a portable radio terminal, and more particularly to an improvement of a small portable radio terminal.
2. Description of the Related Art
In recent years, portable radio terminals have been minimized and lightened for the improvement of portability, thus a small antenna unit has been needed. Then, a whip antenna which can be contained in the main body when carrying, and pulled out from the main body when talking has been developed.
The early portable radio terminals would use the simple whip antenna. However, the whip antenna had a demerit as follows: While the antenna would operate as a monopole antenna when the portable radio terminal is used by pulling out from the main body, the antenna would not generate a sufficient gain when contained in the main body. It is because that the antenna contained in the main body was arranged close to the ground, therefore, the input impedance increased and almost could not be matched.
Then, to improve the gain when the antenna is contained, a top-loading whip antenna in which a helical antenna is electrically connected to the upper end of linear antenna has been utilized. When operating this whip antenna by pulling the antenna out from the main body when in use, radiowaves can be radiated both from the helical antenna and the linear antenna, and when the antenna is contained in the main body, radiowaves can be radiated mainly from the upper end of the helical antenna.
However, this whip antenna has a linear antenna which would not contribute to radiation of radiowaves when contained. The linear antenna should be operated as an open stub. The open stub affects input impedance of the antenna and causes the matching to be slightly disturbed depending on the distance from a substrate in the containing space. Moreover, if the shield is not perfect, the linear antenna contained in the main body picks up a signal, and also a signal enters the shield. The whip antenna had some problems as described above and it could not be said that the construction of this whip antenna was fine.
Then, an antenna unit in which a linear antenna is electrically disconnected when contained was developed. The conventional example of the antenna unit will be described with FIGS. 1 and 2. FIG. 1 shows the state where a linear antenna is contained, and FIG. 2 shows the state where the linear antenna is pulled out. However, the general view of the portable radio terminal is omitted and only the neighborhood of the antenna unit will be described.
The main body 2 of the portable radio terminal 1 is made of a nonmetallic material, as which a circuit board 3 necessary as radio terminal is contained. Various circuits such as a transmitting/receiving circuit 4, and an antenna matching circuit 5, are mounted on the circuit board 3.
The transmitting/receiving circuit 4 and the antenna matching circuit 5 mounted on the circuit board 3 have a function as follows: The transmitting/receiving circuit 4 generates a transmitting signal in the form of the predetermined signal and demodulates a receiving signal received through the antenna unit 6. The antenna matching circuit 5 matches a characteristic impedance from the transmitting/receiving circuit 4 with the input impedance of the antenna unit 6, feeds the transmitting signal supplied from the transmitting/receiving circuit 4 to the antenna unit 6 via a metallic feeding spring 7, and feeds the receiving signal received with the antenna unit 6 to the transmitting/receiving circuit 4.
The antenna unit 6 is composed of two antennas, i.e., a spiral antenna 8 in which a conductor is formed in a spiral, and a linear antenna 9 in which a conductor is formed in a straight line. The antenna unit 6 is attached to the main body 2 by screwing the antenna supporting metal fitting 11 in the antenna unit 6.
The spiral antenna 8 is always connected electrically and mechanically to the antenna supporting metal fitting 11 and always fed electric power via the antenna supporting metal fitting 11, an antenna attaching metal fitting 10, and the feeding spring 7 in succession. The circumference of spiral antenna 8 is covered with a nonconductive antenna cover 12 to avoid touching the user's body.
The linear antenna 9 can be slid in the spiral antenna 8 in the direction of the center axis of the spiral (it is shown by an arrow "a" in FIG. 1), so that the linear antenna 9 can be contained/pulled out. The circumference of linear antenna 9 is covered with a nonconductive antenna cover 13 to avoid touching the user's body when the user pulls out the antenna. The upper end of antenna cover 13 is extended upward and the section is formed in a T-shape so that when the linear antenna 9 is contained, it can be prevented from falling the portable radio terminal 1. The T-shape unit has a function as a knob when the linear antenna 9 is pulled out (hereinafter, it is referred to as a containing stopper/knob 14).
A metallic stopper in pulling out 15 of which the section is formed in convex is attached to the lower end of linear antenna 9, to be mechanically and electrically connected. The stopper in pulling out 15 has a function to protect the linear antenna 9 from becoming dislodged from the main body 2 when the linear antenna 9 is pulled out. Furthermore, the stopper in pulling out 15 is pinched with the antenna supporting metal fitting 11 when the linear antenna 9 is pulled out to electrically connect the linear antenna 9 to the feed spring 7 via the antenna supporting metal fitting 11 and the antenna attaching metal fitting 10 in succession.
In the antenna unit 6 having the structure described above, as shown in FIG. 1, when the linear antenna 9 is contained, the bottom of containing stopper/knob 14 which is a part of the antenna cover 13 is pinched with the antenna supporting metal fitting 11, thus the linear antenna 9 is fixed in the contained state. At this time, the linear antenna 9 is connected to the antenna supporting metal fitting 11 via the nonconductive antenna cover 13, so that the linear antenna 9 is electrically disconnected from the feeding spring 7. As a result, the linear antenna 9 would not operate as an antenna.
On the contrary, the spiral antenna 8 is always electrically connected to the antenna supporting metal fitting 11, and electrically connected to the feeding spring 7 via the antenna supporting metal fitting 11 and the antenna attaching metal fitting 10 in the main body 2. Thereby, the spiral antenna 8 would operate as the helical antenna grounded by the ground conductor of the circuit board 3 and the shield case.
In this manner, in the antenna unit 6, the linear antenna 9 would not operate because the linear antenna 9 is electrically disconnected when the antenna is contained. Thereby, the problem as described above can be solved and fine antenna characteristic can be obtained.
As shown in FIG. 2, when the antenna is pulled out, the stopper in pulling out 15 provided at the lower end of linear antenna 9 is pinched with the antenna supporting metal fitting 11 so that the linear antenna 9 is fixed in the pulled out state. At this time, the linear antenna 9 is electrically connected to the antenna supporting metal fitting 11 via the metallic stopper in pulling out 15, and electrically connected to the feeding spring 7 via the antenna supporting metal fitting 11 and the antenna attaching metal fitting 10 in the main body 2. Thereby, the linear antenna 9 would operate as the monopole antenna grounded by the ground conductor of the circuit board 3 and the shield case. Since the spiral antenna 8 is always fed electric power, the spiral antenna 8 would operate as an antenna also when the antenna is pulled out. However, the linear antenna 9 mainly operates and the spiral antenna 8 operates as a support antenna in this case.
In this manner, when the antenna is pulled out, both the linear antenna 9 and the spiral antenna 8 operate, and when the antenna is contained, the linear antenna 9 is electrically disconnected and only the spiral antenna 8 operates. Thereby, in the antenna unit 6, fine antenna characteristic can be obtained when the antenna is contained comparing with the aforesaid top-loading antenna.
By the way, in the antenna unit 6 as described above, although antenna characteristic when the antenna is contained can be improved by electrically disconnecting the linear antenna 9 when in contained, it was insufficient as an antenna unit because there has been a problem when the antenna is pulled out as follows:
As described above, when the antenna is pulled out, the linear antenna 9 mainly operates and the spiral antenna 8 operates as support. However, since the operational frequency band of spiral antenna 8 is the same as the operational frequency band of linear antenna 9 at this time, the antenna characteristic of mainly operating linear antenna 9 would be disturbed by the spiral antenna 8, thus fine antenna characteristic could not be obtained when the antenna is pulled out.